gb'sfx4,

It is a common problem in an over-boost situation. Sounds like the vanes may be sticking (common on the older designed turbo) or one of the senors is bad that controls the VV turbo.

You will probably need to take it in and have the tech test it to diagnose the problem and fix it. On a side note this question will get a larger response in the 6.0 section a couple of forums down.

Hope this helps